The present invention relates to a water-soluble flame retarder and a process for preparing the same. More particularly the invention relates to a water-soluble flame retarder comprising a high condensed ammonium phosphate and a mixture of urea compounds produced from urea in the presence of a condensed ammonium phosphate, and to a process for preparing the water-soluble flame retarder by the condensation reaction of urea and phosphoric acid.
It is well known that a condensed ammonium phosphate is useful as a flame retarder. There are put on the market at present the condensed ammonium phosphates ranging from low condensed ammonium phosphates having a degree of condensation of not more than 6 to high condensed ammonium phosphates. Various processes for preparing condensed ammonium phosphate, e.g. processes by a liquid phase reaction, vapor phase reaction, vapor-liquid phase reaction and vapor-solid phase reaction are known, and at present the condensed ammonium phosphate is generally produced by a vapor phase reaction at a high temperature under a pressure. In these reactions, phosphoric anhydride and gaseous ammonia are mainly employed as the phosphoric acid and ammonia sources, and water, ammonium dihydrogenphosphate, diammonium hydrogenphosphate or phosphorus pentoxide is also employed as auxiliary agents for condensation. In these processes, however, the reaction must be conducted under severe conditions, i.e. at a high temperature under a pressure and causes a problem such as corrosion of apparatus materials. Also, the product is obtained in the form of a mixture of water-soluble low condensed ammonium phosphates and water-insoluble high condensed ammonium phosphates, since it is difficult to control the degree of condensation. Therefore, in case of employing the condensed ammonium phosphate so produced as a flame retarder in a usual treatment manner such as impregnation or coating, troubles arise such as difficulty in preparing an aqueous solution of a flame retarder in concentrations desired in the treatment, ununiformity of impregnation onto the surface and inside a material to be treated and complication of treatment steps. Also, the higher the degree of condensation, the larger the flame retarding effect. However, since the high condensed ammonium phosphate is water-insoluble, the troubles as mentioned above are noticeable particularly when employing a high condensed ammonium phosphate or a condensed ammonium phosphate containing large quantities of a high condensed ammonium phosphate as a flame retarder. Therefore, in the treatment for imparting flame retardancy to fibers, papers, boards, woods and felts in which an aqueous solution of a flame retarder is employed by means of impregnation, coating, spraying, etc., ammonium dihydrogenphosphate, diammonium hydrogenphosphate or a water-soluble condensed ammonium phosphate having a degree of condensation of not more than 4 has been generally employed as a flame retarder.